Co-based thin-film longitudinal metal media, CoP, CoNiCr, CoNiPt, CoCrTa, and CoCrPt, are gaining increased importance for high-density hard-disk recording due to their relatively large magnetization, high coercivity, and smooth surface as compared to the conventional Co-modified iron oxide particulate media. The achievable linear density in longitudinal recording medium is limited by the demagnetizing field associated with the recorded bits (transitions) but the recorded-bit cell length can be made shorter with a thin medium possessing high coercivity (M. L. Williams and R. L. Comstock, in Magnetism and Magnetic Materials, 1971 (Chicago), AIP Conf. Proc. No. 5, edited by D. C. Graham and J. J. Rhyne (AIP, New York, 1972), p. 738). Also, media noise in thin films, which is attributed mainly to inhomogenieties, grain size and intergranular exchange coupling, (T. Chen and T. Yamashita, IEEE Trans. Magn. MAG-24, 2700 (1988) must be minimized by a suitable microstructure control in order to increase the usable limit of the linear density.
Currently, most of the practical low-noise media possess coercivities of less than 1500 Oe. Coercivities of greater than 1500 Oe have been reported for Co alloys containing Pt up to 45 wt. %. (T. Yogi, C. Tsang, T. A. Nguyen, K. Ju, G. L. Gorman, and G. Castillo, IEEE Trans. Magn. MAG-26, 2271 (1990); J. K. Howard, J. Appl. Phys. 63, 3263 (1988)) Recently, for CoNiCr films coercivity as high as 2300 Oe has been reported but no recording properties are reported. (N. Tani, M. Hashimoto, M. Ishikawa, Y. Ota, K. Nakamura, and A. Itoh, IEEE Trans Magn MAG-26, 1282 (1990) In the future as the head-medium spacing and read-back gap are reduced, significant increases in linear recording densities will be possible if media coercivities are increased. Of course, this can only be accomplished if media noise can be further decreased by maintaining very fine magnetically noncoupled grains. The present invention describes a film structure and process variables necessary to obtain thin (&lt;50 nm) CoSm films by sputtering with high coercivities and with very fine grains or an amorphous structure. Cobalt samarium alloys exhibit high magnetic anisotropy in both the amorphous and crystalline states, (F. J. Cadieu, T. D. Cheung, L. Wickramasekara, N. Kamprath, H. Hegde, and N. C. Liu, J. Appl. Phys. 62, 3866 (1987); E. A. Nesbitt and J. H. Wernick, Rare Earth Permanent Magnets (Academic, New York, 1973) and therefore it may be easier to prepare thin CoSm films with higher in-plane coercivities than is possible with simple transition-metal Co-based alloys. Evaporated amorphous CoSm thin films with coercivities &lt;700 Oe have been studied previously and longitudinal recording properties have been reported. (M. Gronau, H. Goeke, D. Schuffler, and S. Sprenger, IEEE Trans. Magn. MAG-19, 1653 (1983); V. Kullmann, E. Koester, and C. Dorsch, IEEE Trans. Magn. MAG-20, 420 (1984); N. R. Belk, P. K. George, and G. S. Mowry, IEEE Trans. Magn. MAG-21, 1350 (1985)). Because of square hysteresis loops (S=1, S*=1), the noise produced by these films is high and is believed to be due to a zigzag domain-wall structure at the bit transition. Previous investigations of other Co-based alloys such as CoP, CoNiCr, CoCrTa, and CoNiPt have shown that using an underlayer of nonmagnetic polycrystalline such as Cr or W (R. Ranjan, J. Appl. Phys. 67, 4698 (1990)) helps increase coercivity by controlling grain size and morphology, improves the in-plane magnetization by providing an appropriate crystallographic texture, and reduces media noise by providing a magnetically isolated or voided microstructure. To further reduce media noise and to have a high M.sub.r t product, a multilayer media structure has been developed for CoCrTa (D. J. Sellmyer, D. Yang and J. A. Christopher, J. Appl Phys. 67, 9710 (1990) and CoCrPt (M. M. Yang, S. E. Lambert, J. K. Howard, C. Hwang, Laminated CoPtCr/Cr Films for Low Noise Longitudinal Recording, 5th Joint MMM-Intermag Conference, Jun. 18, 1991, Pittsburgh, Pa.) The present invention involves an underlayer and an overlayer in the structure of a film comprised of an alloy of at least Co and Sm.